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2 cos 75° cos 15° =____________________(a) \(\frac{\sqrt{3}}{\sqrt{2}}\)(b) \(\frac{1}{2}\)(c) \(\frac{\sqrt{3}}{2}\)(d) \(\frac{1}{\sqrt{2}}\)I have been asked this question in a national level competition.Question is from Trigonometric Functions of Sum and Difference of Two Angles-2 in portion Trigonometric Functions of Mathematics – Class 11

Answer» RIGHT choice is (B) \(\FRAC{1}{2}\)

Easiest explanation: We know, 2 cos x cos y = cos (x + y) + cos (x – y)

2 cos 75° cos 15° = cos (75°+15°) + cos (75°-15°)

= cos 90° + cos 60°

= 0 + 1/2 = 1/2.


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